Conventionally, a fiber reinforced composite (FRP) comprising reinforced fibers and a matrix resin has been applied to many fields, such as aircraft members, automobile members, ship members, and sporting goods members since it excels in mechanical properties, such as strength, rigidity, and impact resistance though it is lightweight.
As a matrix resin of a fiber reinforced composite material, a thermosetting resin, such as an epoxy resin, an unsaturated polyester resin, a vinyl ester resin, a phenolic resin, a cyanate resin, and a bismaleimide resin, is used in many cases, and an epoxy resin is most often used especially in a field where a comparatively high performance is required.
An epoxy resin is a typical thermosetting resin and is widely used in many fields, such as a casting resin, a coating material, adhesives, and an IC package, in addition to a matrix resin for a composite material on ground that a cured product thereof has excellent in mechanical properties such as a flexural modulus and tensile elongation, a excellent heat-resisting property, excellent chemical resistance, and small shrinkage by curing, and the like.
However, above-mentioned thermosetting resins (cured products) which are represented by an epoxy resin and which are used as a matrix resin of a composite material are poor weatherability because they are unstable against light, especially ultraviolet rays, receive a photooxidation easily to change color and get deterioration of a gloss and mechanical properties. Therefore, they are difficult to apply for uses without painting or with transparent paint under environment where it is exposed to ultraviolet rays, such as outdoors.
However, a fiber reinforced composite is not only excellent in mechanical properties, but it has been attracting an attention for some time that it is also worthy in aesthetics since the color and forms (for example, textile texture, and the like) of reinforcing fibers are in sight from the outside when a transparent matrix resin is used. Therefore, it has been desired that it is used as a structural material with an aesthetic value without painting or with transparent painting.
However, there has been a problem because coloring by ultraviolet rays might spoil aesthetic properties greatly in case a conventional thermosetting resin was used. Although there was actually an example using a fiber reinforced composite material with transparent painting as an automobile outer panel and an aerodynamic member, it had a weak point of coloring comparatively for a short period of time.
By the way, a fiber reinforced composite using an acrylate resin (thermoplastic resin) which is excellent in weatherability although insufficient in mechanical properties as a matrix resin is already put in practical use as material for decorative part for automobile interiors [“Carbon fiber in Japanese Cars”, High-Performance Composites, vol.7, issue 3, p.17 (1999)]. However, an acrylate resin could be used only as material for pure decorative part, in spite of excellent weatherability, since it had a low elastic modulus and a composite material thereof had inadequate mechanical properties (especially a compressive strength, flexural strength) and it is not suitable for manufacture of a fiber reinforced composite material with a high content of reinforcing fibers because it is a thermoplastic resin.
As a method of obtaining a fiber reinforced composite material having both mechanical properties and weatherability, Japanese Unexamined Patent Publication No. 2002-105789 discloses a method using a weatherproof matrix resin near the surface and a matrix resin having excellent mechanical properties in the remaining portion. However, this method has a weak point that a manufacturing process is rather complicated, and a method by which the same performance is obtained by a single matrix resin has been desired.
Furthermore, an epoxy resin has important applications requiring weatherability, such as a coating material and adhesives, other than a matrix resin for composite materials, and an epoxy resin having a high weatherability has been desired.
In the field of plastics, generally, an ultraviolet absorbent, such as a benzotriazol derivative and a benzophenone derivative, is added for improving weatherability. However, as to a typical epoxy resin, addition of an ultraviolet absorbent has no effect and rather there was even a case that it increased a color change. Therefore, in the field of an epoxy resin, addition of an ultraviolet absorbent has scarcely been studied and there has been little information about an effective combination of an epoxy resin and an ultraviolet absorbent though it was a general approach of weather-resistance improvement of plastics.
There are some attempts to improve weatherability of an epoxy resin with no ultraviolet absorbent added.
“Introduction to Epoxy Resin” written by Soichi Muroi and Hidekazu Ishimura (Kobunshikankokai, 1988) discloses, on page 52, a composition comprising dodecahydro(bisphenol A) diglycidyl ether and Epomate B-002 (modified amine), and it discloses that a cured product thereof is excellent in a gloss retention after exposure to light.
And Japanese Unexamined Patent Publication No. 2000-143939 discloses a composition comprising dodecahydro(bisphenol A) diglycidyl ether, a cycloaliphatic epoxy resin and a carboxylic anhydride or a cationic initiator, and it discloses that a cured product thereof is excellent in a gloss retention after exposure to light.
However, a cured product of these resin compositions has inferior weatherability than other weatherproof plastics such as an acrylate resin although it has remarkably improved weatherability regarding gloss retention in comparison with an ordinary epoxy resin. And these prior art documents describe nothing about color change, or about addition of an ultraviolet absorbent.
The present invention aims to provide an epoxy resin composition excellent in mechanical properties and also in weatherability, especially with color change after exposure to light being small. Furthermore, the present invention aims to provide a fiber reinforced composite material excellent in mechanical properties and also in weatherability, especially with color change after exposure to light being small, and having an aesthetic properties originated in a reinforcing fiber tissue, by the use of a single thermosetting resin.